A variety of known systems exist with the purpose of reducing flammability in aircraft fuel tanks. Such systems may be known by a number of designations including, but not limited to, On-Board Inert Gas Generation System (OBIGGS), Nitrogen Generation System (NGS), Flammability Reduction System (FRS), Fuel Tank Inerting System (FTIS), etc. However, a commonality among the systems involves reducing the oxygen content of fuel tank ullage by feeding an oxygen-depleted gas into the fuel tank. Fuel tank ullage is a region in the tank that often contains evaporated fuel (fuel vapor). Often, the systems produce nitrogen-enriched air (NEA), also called oxygen-depleted air (ODA), as the oxygen-depleted gas. The NEA reduces flammability of the fuel tank ullage since ullage with lower oxygen content is less flammable.
Inerting systems used to produce nitrogen-enriched air may rely on diffusion through polymer membranes, or other separation mechanisms, to remove oxygen. In systems with polymer hollow fiber membranes, compressed air enters the bore of the polymer hollow fiber and oxygen permeates through the walls of the polymer hollow fiber. The oxygen-enriched permeate is collected and exhausted overboard. The remaining nitrogen-enriched retentate flows through the bore and is collected at the air separation module product gas outlet for distribution to aircraft fuel tanks. Unfortunately, service life of the air separation module and the system operating conditions may be limited by, among other factors, the polymers used in construction of the air separation module. Accordingly, increased reliability of air separation modules is desirable.